I believe a small number of people will do something stupid, like the human race always seems to do when shit is supposed to happen (according to some off source). There may be a few people commit suicide (not in the masses just one here or there), but that will actually benefit humanity as a whole. It keeps another ignorant person from reproducing.
Nothing will happen, at all. Though I do expect an end of the world at some point with the return of Christ, but who knows when that'll happen. The December 21'st thing, I don't know how anybody came to the conclusion that the Mayan Calendar running out somehow meant anything but whatever.